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Ultrafast electron microscopy:Goals, achievements, and challenges


Ultrafast electron microscopy (UEM) is a technique that aims to combine the high spatial (sub-nanometer) resolution of electron microscopy with the high temporal resolution (sub-picosecond) afforded by today’s ultrashort pulse laser systems.  To date, imaging dynamic transmission electron microscopy (DTEM) has realized a space-time resolution of close to 10nm.ns in a single-shot regime and about at ~100MHz (multi-shot) repetition rates, albeit with only a single electron per pulse.  The performance of both of these DTEMs has been shown to be limited by space-charge effects; in the spatial dimension for the former ~10ns-pulsed instruments and predominantly in the temporal dimension for the latter UEM driven by a femtosecond laser radiation source.

In this talk, I will present a rationale for dealing with space-charge effects in sub-picosecond UEMs that should result in significantly imaging space-time resolution.  The idea is to segregate the space-charge effects into the temporal domain through the use of ‘disk-like’ pulses (spatial width >> temporal length) to allow for their compensation (by pulse compression) using an RF cavity.  The resultant effective removal of space-charge effects from the transverse spatial dimensions could then allow the emittance of the ultrafast photo-electron gun to be reduced through radiative control of the photoemission mechanism.  Dielectric-loaded TM 010 RF cavities and plasmon-enhanced photoemission from nano-patterned photocathodes provide two attractive candidate technologies for a DTEM with imaging resolution.